1. Field of the Invention
This invention relates to precipitated amorphous silica, and processes for making it. The precipitated silica is especially well-adapted for use in dentifrices containing cetylpyridinium chloride.
2. Description of the Related Art
Modern dentifrices often contain an abrasive substance for controlled mechanical cleaning and polishing of teeth, and optionally a chemical cleaning agent, among other common ingredients, such as humectants, flavors, therapeutic ingredients, such as an anticaries agent, rheology control agents, binders, preservatives, colors, and sudsing agents, among others. Oral care products also often contain therapeutic agents, such as anti-microbial agents. Cetylpyridinium chloride (“CPC”) is an anti-microbial agent used for this purpose, such as in mouthwashes and toothpastes. There is an increased desire among dentifrice manufacturers to incorporate anti-microbial agents in dentifrice applications for the control of malodor and/or other therapeutic action, with CPC being one of the more favored. It is cost effective and generally recognized as safe. By contrast, some alternative anti-microbial agents currently being used in dentifrices have come under increasing scrutiny for possible contribution to the increased resistance of some bacterial strains to antibiotics. CPC is not considered to contribute to this health problem.
CPC is a cationic (“positively”) charged compound. CPC's antimicrobial action is generally understood to result from its ability to bind to anionically (“negatively”)-charged protein moieties on bacterial cells present in the mouth. This CPC attachment mechanism results in a disruption of normal cellular function of bacteria and contributes to the prevention of plaque formation and other bacterial actions.
A problem encountered in CPC usage in dentifrices has been that CPC tends to indiscriminately bind to negatively-charged surfaces. In particular, co-ingredients of toothpaste formulations having negatively-charged surfaces also may bind to CPC before it performs any antimicrobial action. Once bound to these nontargeted surfaces, the CPC is generally unavailable to perform any meaningful antimicrobial action.
In this regard, silica is often used as an abrasive in dentifrices. For instance, silica's abrasive action is used for pellicle removal from teeth. Most conventional silicas used in dentifrices have negatively-charged surfaces. Consequently, CPC adsorbs onto such conventional silica powders. For reasons explained above, the adsorption of CPC upon silica or other co-ingredients of the dentifrice is highly undesirable.
U.S. Pat. No. 6,355,229 describes a CPC compatible dentifrice formulation containing guar hydroxypyropyl-trimonium chloride. The guar complex has a higher affinity toward binding to negatively-charged species. It preferentially binds to anionic components leaving CPC free to bind to plaque.
U.S. Pat. No. 5,989,524 describes a silica that is compatible with flavors obtained by treating the surface of the silica originating from the reaction of an alkali metal silicate with an inorganic or organic acidic agent with the aid of an organic compound capable of developing hydrogen or ionic bonds with the Si—OH silanol groups or the SiO− anionic groups at the silica surface. The organic agent can be added to the silica in the form of slurry before or after salts are removed, or can be sprayed on to dry silica.
A number of patent publications describe processes for making composite synthetic silica particles, including the following.
U.S. Pat. No. 2,731,326 describes a process of preparing xerogels in which a silica gel is stabilized so that the pores of the gel do not collapse upon drying. It involves a two-stage precipitation process where in the first stage silica gel is formed, and in the second stage a layer of dense amorphous silica is formed over the gel particles in order to provide sufficient reinforcement such that the pores do not collapse upon drying. The gel particles have a particle size in the range of 5 to 150 millimicrons (nm), and preferably have an average diameter of from 5 to 50 millimicrons. The resulting reticulated particles can be dewatered and dried into powder form. The '326 patent states that when silica particles have a specific surface area of greater than 200 m2/g, it is preferred to replace the water with an organic liquid, and then dehydrate the silica particles. The '326 patent describes silica products with preferred specific surface areas 60 to 400 m2/g. The '326 patent indicates little advantage is obtained in carrying the process of accretion to an extreme. The preferred products of the '326 patent process of accretion are limited so that the original dense ultimate units of the aggregates do not lose their identity and the original aggregates structure is not obscured.
U.S. Pat. No. 2,885,366 describes a process used to deposit a dense layer of silica over particles other than silica.
U.S. Pat. No. 2,601,235 describes a process for producing built-up silica particles in which a silica sol heel is heated above 60° C. to make nuclei of high molecular weight silica. The nuclei is mixed with an aqueous dispersion of active silica made by acidulating alkali metal silicate, and the mixture is heated above 60° C. at a pH of 8.7 to 10, such that active silica accretes to the nuclei.
U.S. Pat. No. 5,968,470 describes a process to synthesize silica having controlled porosity. It involves the addition of silicate and acid to a solution of colloidal silica with or without an electrolyte added (salt). The porosity can be controlled based upon the amount of colloidal silica added in the first step of the reaction. Silica with BET surface areas ranging from 20 to 300 m2/g, CTAB specific surface areas from 10 to 200 m2/g, oil absorption (DBP) ranging from 80 to 400 m2/g, pore volumes from 1 to 10 cm3/g, and mean pore diameters from 10 to 50 nm could be synthesized. The intended use of materials produced by this process is in the paper and catalysis marketplace.
U.S. Pat. No. 6,159,277 describes a process for the formation of silica particles with a double structure of a core of dense amorphous silica and a shell of bulky amorphous silica. A gel is formed in a first step. The gel is then aged, wet pulverized, and then sodium silicate is added in the presence of an alkali metal salt in order to form amorphous silica particles on the surface of the milled gel particles. The resultant double structure silica material has an average particle diameter of 2 to 5 micrometers and a surface area of 150 to 400 m2/g. The resultant material is said to have improved properties for use in as a delustering agent in paint and coatings.
Patent publications that describe use of silicas in dentifrice or oral cleaning compositions include the following.
U.S. Pat. No. 5,744,114 describes silica particles adopted for formulation into dentifrice compositions having a unique surface chemistry as to be at least 50% compatible with zinc values, and have a number of OH functions, expressed as OH/nm2, of at most 15 and a zero charge point of from 3 to 6.5. The '114 patent describes a process of preparing silica particles by the reaction of silicate with an acid to form a suspension or gel of silica. The gel/suspension is then separated, washed with water and treated with acid to adjust the pH below 7.
U.S. Pat. No. 5,616,316 describes silica that is more compatible with customary dentifrice ingredients. In addition to many other ingredients, cationic amines are mentioned.
Another problem associated with usage of conventional silicas in dentifrices is that they often have flavor compatibility problems. That is, the conventional silicas tend to interact with flavorants included in the same dentifrice in a manner that creates off-flavors, making the product less palatable. This off-flavor problem accompanying use of some conventional silicas in dentifrices is highly undesirable from a consumer satisfaction standpoint.
A need exists for silicas that can be used together with anti-microbial agents such as CPC in oral cleaning compositions such as dentifrices without impairing the respective functions of either ingredient. Silicas that are more flavor compatible are also in need. In general, the silica disclosed in this invention may be useful whenever it is desirable to limit the interaction of the silica particulate with desirable additives and components found in dentifrice formulations. The present invention meets these needs and others as will become readily apparent from the disclosure that follows.